Products designed around thermoplastic polyester resins are in great demand because of the tremendous versatility of these resins and their inherent properties. Polyethylene terephthalate (PET) with its slow crystallization rate has largely been used in packaging: however, nucleating agents have been used to increase the crystallization rate, and post-molding heat treatments (annealing) can be used to crystallize molded articles made of PET. The crystallized PET tends to higher tensile strength, stiffness, and heat resistance compared to uncrystallized PET.
Because of the unique engineering properties of the PET resin and in a continuing search to improve the overall property spectrum of molded PET, the resin has often been blended with numerous other resins. Additionally, specific additives have also been incorporated to increase the general level of the impact strength of the PET-containing blends as well as the consistency of the impact properties in molded articles.
Illustrative of specific additives or impact modifiers that have been proposed to be used with thermoplastic polyester resins are the family of modifiers comprising polyalkyl acrylates, methacrylates, and/or ethacrylates (Brinkmann et al: U.S. Pat. No. 3,591,659) typical of which is Acryloid KM-330, a polyacrylate resin sold by Rohm & Haas Co.
Certain rubber resin graft copolymers have also been proposed to affect impact resistance of polyester resins. In U.S. Pat. No. 3,919,353 (Castelnuovo et al) EPDM grafted with common ethylenically unsaturated monomers is disclosed to be an effective impact modifier. Polybutadiene is taught to be the preferred elastomer. All of the examples use polybutadiene grafted with styrene-methyl methacrylates and the elastomer is crosslinked.
One of the resin blends preferred for its overall balance of molded properties is that comprising polyesters and polycarbonate (PC) resins. It should be noted that the impact strength at room temperature of most polycarbonates is relatively high. For example, the Notched Izod Impact at Room Temperature (NIRT)(ft-lb/in.) of numerous commercial polycarbonates such as LEXAN 103 (trademark of General Electric) is often in the range of from about 16 to 20 ft-lb/in. However, to obtain maximum chemical resistance of polyester/polycarbonate blends, it is desirable to keep the polycarbonate content low. Furthermore, polycarbonate resin is susceptible to mechanical stress cracking which can be exacerbated by exposure to unfavorable environments such as the presence of solvents or solvent vapors. This tendency to stress crack provides an additional reason to utilize as little polycarbonate resin as possible in these polyester blends.
When adding impact modifiers to polyethylene terephthalate/polycarbonate blends, susceptibility to the effects of ester interchange on the final blend properties must also be taken into account. Blends produced under high shear conditions may have unpredictable properties due to reaction (ester interchange) between the polyester and polycarbonate components. The ester interchange reaction can be minimized, of course, by the use of inhibitors and/or by lowering the shear level.
Various blends based on polyester resin, polycarbonate resin and graft copolymer rubber-resin materials have heretofore been proposed (e.g., U.S. Pat. Nos. 3,591,659, 3,919,353; 4,022,748; 4,034,013; 4,034,016; 4,044,073; 4,096,202; 4,260,690 and 4,280,949.
U.S. Pat. No. 4,257,937 teaches the impact modification of polyester/polycarbonate blends via a polyacrylate resin such as Acryoid KM-330 and 7709-XP (trademarks of Rohm & Haas Co.)
U.S. Pat. No. 4,172,859 (Epstein) discloses blends of polyesters, polycarbonate and an impact modifier which can comprise various monomers, combinations of which can include EPDM.
U.S. Pat. No. 4,493,921 represents another step in the continuing effort to improve the level of performance of polyester-type resin/polycarbonate-type blends via the addition to polybutylene terephthalate (PBT)/PC blends of an impact modifier comprising an ethylene-propylene-nonconjugated diene (EPDM) rubber spine grafted with either a styrene/acrylonitrile monomer mixture, a styrene/methyl methacrylate monomer mixture or a methyl methacrylate monomer mixture.
U.S. Pat. No. 4,547,536 describes the preparation of PET/PE molding resin from soda bottle regrind via the incorporation of a dessicant, mineral oil, and a block copolymer impact modifier. Unfortunately, the blends described by this patent have rather low impact strengths, especially Notched Izod Impact values.
One of the art recognized difficulties realized in the impact-modification polyester art is the unpredictability of the modifier effect, i.e., a modifier that enhances the properties of one polyester such as polybutylene terephthalate (PBT) or a PBT resin blend, may have little property modification in a different polyester or polyester blend. For example, certain polyacrylate modifiers such as Acryloid KM-330, while capable of significantly improving the impact properties of a PBT/PC resin blend, will have much less effect on a PET/PC blend.
About 20% of all plastic bottles produced in the Unites States are made from PET and the recycling of these bottles prove to be a major source of PET. These bottles are conventionally prepared from a polyethylene terephthalate resin with a base cup of high density polyethylene (HDPE) resin which comprises about one-third of the weight of the PET.
Separation plants (recyclers) often crush the bottles as received, i.e., together with the base cups, finely grind the material, and separate the components, paper (from labels), aluminum (from caps), PE (from base cups) and PET, by various methods including air separation, washing, and flotation. Since the presence of PE in the PET is deleterious to the overall properties of the PET resin, especially the impact strength of the resin, extensive separation procedures are used to obtain a PE-free PET stream for most recycle molding end-uses.
It would be highly desirable to improve the property spectrum, especially the impact strength of PET/PE resin blends and thus obviate the need for the additional separation now deemed necessary by recyclers and PET molders.